Seasonal dynamics in RNA:DNA ratios of marine microbial communities in a high latitude estuary
Forfatter
Prytz, Ragnhild Helene GrøholtSammendrag
Microorganisms are key players in marine ecosystems, as they form the basis of food webs and drive biogeochemical cycling. Global warming increases sea surface temperatures and the frequency of marine heatwaves, but the effect on marine microbes is still not well understood. To predict future changes of marine ecosystems, it is therefore important to understand the direct impact of increased temperatures on microorganisms. A recent study on soil microorganisms reported a reduction of ribosomes with increasing temperature. Ribosomes are key cellular complexes responsible for protein biosynthesis, and a reduction of ribosomes was proposed as a universal physiological response of microorganisms to warming.
The objective of this thesis was to adopt and further develop a nucleic acid extraction protocol used to investigate temperature effects on soil microorganisms, to be applied on marine microorganisms. This method allowed me to investigate the effect of temperature on the RNA:DNA ratios of microbial communities in the high latitude estuary, Håkøybotn (Tromsø, Norway). The RNA:DNA ratios served as a proxy for cellular ribosome contents, and I tested the hypothesis that ribosome contents of marine microorganisms are lower in summer (at higher temperatures) compared to winter (at lower temperatures). Such a reduction of ribosomes with increased temperature could lead to alterations of energy transfer efficiency in marine food webs and biogeochemical cycling. The microbial communities and environmental parameters such as temperature, nitrate, phosphate, and silicate concentrations, Chlorophyll a, salinity and pH were sampled over eight months, from June 2024 to January 2025. In total, 19 samplings were conducted. The water samples were sequentially filtered to separate microorganisms into four size fractions (10 – 90, 3 – 10, 0.2 – 3, and 0.2 – 90 µm) with 6 replicates per fraction. RNA and DNA was extracted from >200 samples (i.e. all 10 – 90 and 0.2 – 3 µm fractions and up to 6 replicates per fraction). Additionally, 18S and 16S rRNA gene amplicon sequencing was performed on selected samples to analyse seasonal microbial community shifts.
Most measured parameters displayed seasonal patterns: Water temperature ranged from 13.3 to 2.6°C, nutrient concentrations were low in summer and high in winter and Chlorophyll a concentrations peaked in August and September. A weak but significant negative correlation was found between temperature and the RNA:DNA ratio for the smallest size fraction (0.2 – 3 µm), indicating that temperature might influence the ribosomal content of marine microorganisms, as hypothesized. However, this patter was not observed in the large fraction (10 – 90 µm), but possibly due to lack of data obtained during the winter month. The observed seasonal shifts in the microbial communities could potentially also have influenced the measured RNA:DNA ratios. However, additional knowledge regarding varying RNA:DNA ratios between taxa is needed to determine a connection between dynamics in RNA:DNA and community composition.
In light of recent temperature records in the Norwegian Sea it is important to gain knowledge on the physiological response of marine microbes and their possible consequences for ecosystem functioning. The developed method was adaptable to changing conditions in the marine ecosystem and could be used for future research on marine microbial communities.
Forlag
UiT The Arctic University of NorwayMetadata
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